The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations, and particularly to polycrystalline diamond compact (PDC) drag bits.
A rotary drill bit of the kind to which the present invention relates comprises a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of preform cutting elements each formed, at least in part, from polycrystalline diamond. One common form of cutting element comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machines from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix.
While such PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations and soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters suffer accelerated wear and bit life can be too short to be commercially acceptable.
Recent studies have suggested that the rapid wear of PDC bits in harder formations is due to chipping of the cutters as a result of impact loads caused by vibration, and that the most harmful vibrations can be attributed to a phenomenon called "bit whirl". ("Bit Whirl--A New Theory of PDC Bit Failure"--paper No. SPE 15971 by J. F. Brett, T. M. Warren and S. M. Behr, Society of Petroleum Engineers, 64th Annual Technical Conference, San Antonio, Oct. 8-11, 1989). Bit whirl arises when the instantaneous axis of rotation of the bit processes around the central axis of the hole when the diameter of the hole becomes slightly larger than the diameter of the bit. When a bit begins to whirl some cutters can be moving sideways or backwards relatively to the formation and may be moving at much greater velocity than if the bit were rotating truly. Once bit whirl has been initiated, it is difficult to stop since the forces resulting from the bit whirl, such as centrifugal forces, tend to reinforce the effect.
Attempts to inhibit the initiation of bit whirl by constraining the bit to rotate truly, i.e. with the axis of rotation of the bit coincident with the central axis of the hole, have not been particularly successful.
Although it is normally considered desirable for PDC drill bits to be rotationally balanced, in practice some imbalance is tolerated. Accordingly it is fairly common for PDC drill bits to be inherently imbalanced, i.e. when the bit is being run there is, due to the cutting, hydraulic and centrifugal forces acting on the bit, a resultant force acting on the bit, the lateral component of which force, during drilling, is balanced by an equal and opposite reaction from the sides of the borehole.
This resultant lateral force is commonly referred to as the bit imbalance force and is usually represented as a percentage of the weight-on-bit since it is almost directly proportional to weight-on-bit. It has been found that certain imbalanced bits are less susceptible to bit whirl than other, more balanced bits. ("Development of a Whirl Resistant Bit"--paper No. SPE 19572 by T. M. Warren, Society of Petroleum Engineers, 64th Annual Technical Conference, San Antonio, Oct. 8-11, 1989). Investigation of this phenomenon has suggested that in such less susceptible bits the resultant lateral imbalance force is directed towards a portion of the bit gauge which happens to be free of cutters and which is therefore making lower "frictional" contact with the formation than other parts of the gauge of the bit on which face gauge cutters are mounted. It is believed that, since a comparatively low friction part of the bit is being urged against the formation by the imbalance force, slipping occurs between this part of the bit and the formation and the rotating bit therefore has less tendency to process, or "walk", around the hole, thus initiating bit whirl.
(Although, for convenience, reference is made herein to "frictional" contact between the bit gauge and formation, this expression is not intended to be limited only to rubbing contact, but should be understood to include any form of engagement between the bit gauge and formation which applies a restraining force to rotation of the bit. Thus, it is intended to include, for example, engagement of the formation by any cutters or abrasion elements which may be mounted on the part of the gauge being referred to.)
This has led to the suggestion, in the above-mentioned paper by Warren, that bit whirl might be reduced by omitting cutters from one sector of the bit face, so as deliberately to imbalance the bit, and providing a low friction pad on the bit body for engaging the surface of the formation in the region towards which the resultant lateral force due to the imbalance is directed.
Experimental results have indicated that this approach may be advantageous in reducing or eliminating bit whirl. However, the omission of cutters from one sector of a PDC bit can have disadvantages, and our co-pending British Patent Application No. 8926688-6 discloses some alternative and preferred arrangements for providing the necessary imbalance in the bit in an arrangement for reducing or eliminating bit whirl. The present invention relates to arrangements for providing the necessary low friction pad on the bit body. The arrangements to be described may provide a low friction pad for use with any method of providing the imbalance force, including but not restricted to those arrangements disclosed in the above mentioned co-pending application.